This invention provides a means by which a guided missile, a homing type missile, e.g., when confronted by two or more targets in formation, can discriminate against all but one of the targets, and can successfully intercept the selected target.
The science of homing guidance systems in missiles has been extensively and successfully practiced for many years in the interceptions and destruction of targets of many shapes, speeds and closing velocities. However, if instead of one target, the threat is a multiplicity of targets, such as air vehicles flying in formation, the probability of successfully intercepting one of the targets is greatly diminished. The usual reaction of a homing system to a multiple target threat is to attack the center of the reflected energy emitted from the multiple target formation. As a result, the missile may fly through the formation and miss all of the targets.
An active or semi-active homing system in a missile derives a target signal by illuminating the target with a narrow beam of radio energy and then comparing the frequency of this radio beam as received by the missile with the frequency of the portion reflected from the target. The difference between the direct beam frequency and the reflected beam frequency is a function of the closing velocity between the missile and the target. This is known as the Doppler effect. If the closing velocity between the missile and the target is zero, the difference or Doppler frequency will also be zero and the missile will never get any closer to the target. In the usual case, there will be an appreciable closing velocity between the missile and the target and a usuable Doppler frequency will result which will be proportional to the closing velocity.
In this case of multiple targets, two factors serve to produce differences between the Doppler frequencies derived for each of the targets. The first is the effect called jockeying which is caused by speed differences among the targets as they attempt to maintain their formation positions. This can produce Doppler difference frequencies of up to several hundred hertz per second. The second cause of Doppler differences among the target returns is produced by the geometrical relationships that occur in that portion of the flight just prior to intercept. This can be illustrated by assuming, for example, that a missile is heading straight for one of the targets in a formation. As the distance between the missile and the target formation decreases, the angular spread among the targets becomes increasingly significant. While the closing velocity between the missile and the target toward which the missile is steering will not change, the closing velocity between the missile and each of the other targets will decrease at an increasing rate and will drop to zero at the interception of the chosen target. Therefore, it can be seen that in order to attain a successful interception of one of a group of targets, a multiple target system must be able to select and derive its guidance from the target having the highest Doppler frequency while suppressing the guidance information of the other targets. The present invention provides a solution to the problem by utilizing the frequency characteristics of the multiple target signals to discriminate against all but one of the targets.